Electromagnetic interference (EMI) is created from everyday natural sources. Additionally there are innumerable sources of man made EMI typically created and radiated by televisions, power transmission lines, ignition systems, fluorescent lightning, radar transmissions, electric car chargers, and computing devices. These sources of EMI radiation challenge the equipment, designers, and engineers to find a solution to keep electronic signals coming to equipment clean and usable, isolated from the negative effects of the ever present EMI.
EMI filters, which are electronic devices having suitable capacitive and inductive characteristics for reducing the onerous EMI, are commonly installed in electronic circuits to achieve this goal.
Filters for the application of interest here are commonly fabricated by prior art methods and typically consist of a discoidal capacitor, feed through filter in a bulk head mount configuration that is placed in a signal path to redirect electromagnetic interference back to its source.
Feed through filters of this type consist of either a capacitor (C-only) or a combination of capacitive and inductive elements arranged in classic filter configurations (LC, Pi, or T). Each of these configurations fits a particular application requirement. The most economical solution is to select the filter with the fewest internal parts that achieves the desired filtering effect.
C-only filters, i.e., filters that consist solely of capacitive elements, can be well-suited for filtering high frequency signals on lines with very high impedance. In at least some practical cases, the attenuation of these devices increases in steps of 20 dB per decade from the filter's cutoff frequency up to the frequency where they reach an attenuation of at least 60 dB. (There are many C filters that provide less performance in practical applications. Many physically-realizable filters cannot achieve 60 dB attenuation.)
The LC type filter can be well-suited for applications in which there are large differences between line and load impedances. (Note that effective LC filters can also be built where both source and load impedances are identical.) These devices consist of a capacitive element, in the same manner as the C-only filter, with the addition of an inductive element connected in series with the capacitor between the input and output terminals. Usually, it is best to install the filter so that the inductive element faces the lower impedance terminal. With respect to the conventional packaging of discoidal capacitor type filters, this means that in some applications it is desirable to have the capacitive element close to the threaded or screw-neck header end of the filter package, while in other cases the reverse is desirable, with the inductive element located on the threaded or screw-neck end.
Unlike conventional leaded capacitors, the discoidal capacitor's co-axial configuration provides at least two advantages. It prevents radiation present at the input end from coupling directly to the capacitor output. This construction also has inherently low self- and mutual inductance and the combination provides excellent shunting of EMI at frequencies approaching 1 GHz. The addition of inductive elements (wire wound coils, toroids or beads) in series with the capacitor increases the impedance of the line, making the filter even more effective.
Pi filters consist of three elements. A series inductive element is positioned between two capacitors which are shunt connected one across the source and one across the load. Pi filters are well-suited for applications where the input and the output impedances are of relatively high value and high levels of attenuation are required. In at least some practical cases, these filters may increase attenuation by 60 dB per decade from the filter cutoff frequency to the frequency where the filter exhibits an attenuation of at least 80 dB. (The capacitors in a Pi filter work by providing a large impedance mismatch relative to the (higher) source and load impedance in the application circuit. There is no requirement that the source and load impedances be similar for a Pi filter to function effectively. Note also that 80 dB attenuation is not an intrinsic property of a Pi filter; many physically realizable filters do not achieve an attenuation approaching 80 dB.)
The T filter is also a three-element device, but this time there are two series inductors connected between the input and output terminals on each side of a single capacitor which is shunt connected across the signal and its return conductors. In at least some practical cases, the T filter performs in much the same manner as a Pi filter, increasing attenuation in steps of 60 dB per decade from the cutoff frequency to the frequency where the attenuation is at least 60 dB. This filter type is selected when both the input and output impedances are low.
Internally the most complicated device, the LL filter consists of two feed through capacitors connected between line and ground interspersed with two inductors connected in series between the input and output terminals. In at least some practical cases, these filters increase in attenuation in steps of 80 dB per decade from the cutoff frequency to the frequency where the attenuation is at least 80 dB. (Note also that 80 dB attenuation is not an intrinsic property of an LL filter; many physically realizable filters do not achieve an attenuation approaching 80 dB.)
Today, most center through-feed, metal enclosure, metal housing, bulkhead or through hole mounted EMI filters for low frequency, high current applications employ at least one discoidal capacitor element, and commonly use X7R ceramic formulations for the capacitor dielectric. It can be cost-effective and has an adequate dielectric constant at normal operating temperatures.
In particular, at least one type of C-only feed through filter uses a feed through capacitor that mounts into an opening of an enclosure wall and is secured by soldering or mechanical means. A conductor passes through the center of the capacitor and the electrical signal is filtered by the capacitor. The noise currents are shunted to the enclosure through the dielectric material of the feed through capacitor. If multiple conductors are needed, multiple feed through capacitors are used.
In one or more enclosures, a typical data storage system includes data moving circuitry and an array of disk drives. Some data storage systems fit within standard-sized equipment cabinets or racks. The data moving circuitry for such data storage systems is typically fashioned into modules called blade servers, or simply blades, which are housed (perhaps in pairs) within enclosures. Such enclosures are commonly available in a variety of standard heights (e.g., 4 U, 3 U, 2 U and 1 U, where U is a standard measure of vertical height in the equipment cabinet and is equal to 1.75 inches in accordance with the EIA-310-D industry standard).
One conventional enclosure (hereinafter referred to as the conventional 4 U enclosure) includes a 4 U chassis, a midplane, two independent blades, two dual-port power supplies, and three pairs of fans. The midplane sits in the middle of the 4 U chassis. The two blades independently plug into the midplane through a rear opening of the 4 U chassis and reside in a stacked manner within the 4 U chassis, one above the other. Similarly, the dual-port power supplies independently plug into the midplane through the rear opening of the 4 U chassis and straddle the two blades along the sides of the 4 U chassis. In particular, a rightside power supply sits right side up within the 4 U chassis, and a leftside power supply sits upside down within the 4 U chassis, thus enabling the manufacturer to connect both dual-port power supplies to the midplane while maintaining a single dual-port power supply design for the 4 U chassis. One power cord to each dual port power supply plugs into the rear face of the power supply through an IEC-C14 appliance coupler mounted on each power supply. Additionally, each pair of fans plugs into the midplane through a front opening of the 4 U chassis.
During operation, the midplane carries DC power supply signals from the two dual-port power supplies to the three pairs of fans and to the two blades (i.e., a first port of each dual-port power supply outputs power supply signals to one blade and a second port of each dual-port power supply outputs power supply signals to the other blade). The power for both the fans and the blades is diode-OR'd or shared to support backup of each other in the event of one DC power supply failing. Each blade of the 4 U enclosure typically includes two microprocessors and performs data storage operations. For example, each blade stores data into and retrieves data from an adjacent array of disk drives on behalf of one or more external host computers. The three pairs of fans pass air over the blades in a shared configuration to thermally maintain the blades within a controlled temperature range.